64 PHYSIOLOGY. 



ribs, and by compressing the abdomen force the viscera up against 

 the relaxed diaphragm, and thus diminish the cavity of the thorax 

 from below. These are the recti, obliqui, and transversi abdominis, 

 the quadratus lumborum, the serratus posticus inferior, the sacro- 

 lumbalis, and the longissimus dorsi. 



If the whole time occupied by a respiratory act, from the begin- 

 ning of one inspiration to the beginning of the next, be represented 

 by 10, the inspiratory movement may be estimated at 5; the expi- 

 ratory at 4 ; while the remaining 1 will be occupied by the period of 

 repose which succeeds expiration. 



The capacity of the lungs varies very much in different indivi- 

 duals. According to Mr. Hutchinson, by this term is meant " that 

 quantity of air which an individual can force out of his chest by the 

 greatest voluntary expiration, after the greatest voluntary inspira- 

 tion." This author has shown that in health, this capacity bears a 

 very constant relation to the height of the individual. Thus the 

 mean " capacity" of 172 males under the height of 5 ft. 8 in. is 220 

 cubic inches, whilst that of 82 males from 5 ft. 11. to 6 ft. is 255 

 cubic inches. For every additional inch of height (from 5 to 6 ft.), 

 8 additional cubic inches are given out by a forced expiration. The 

 exceptions to this occur among stout and corpulent individuals, whose 

 capacity stands lowest. The size of the chest, therefore, is not always 

 a criterion as to the capacity of expiration. Mr. Hutchinson also 

 found that the full expiratory force of a healthy man is commonly 

 one-third greater than his inspiratory force. 



According to the experiments of Mr. Coathupe, about 2 66 J cubic 

 feet, or 460,224 cubic inches of air, pass through the lungs of a 

 middle-sized man in 24 hours ; reckoning the average number of 

 inspirations at 16 per minute, this would give 20 cubic inches as the 

 amount inhaled at each. 



Residuary air. — According to the calculation of Sir H. Davy, 

 the human lung, after the strongest expiration, still contains 35 cubic 

 inches of air, and after an ordinary expiration, 108 cubic inches. 

 This (air) is called the residuary air, and upon it depends the light- 

 ness of the lungs, which enables them to float upon water ; in fact, 

 when once these organs have been distended by a full inspiration, no 

 mechanical, or other power can dislodge the air so as to cause them 

 to sink in water. Upon this circumstance 'was based the hydro- 

 static test of infanticide. The residuary air also gives rise to the 

 tympanitic resonance on percussion. 



In the act of inspiration the air within the lungs is alternately in- 

 creased and diminished in amount ; and thus a regular exchange is 

 secured. This exchange, however, affects only a portion of the con- 

 tained air at a time, and this proportion appears to vary according 

 to the frequency of^ the respiration. Indeed, if it were not for the 

 tendency of gases to mutual diflJusion, the air in the remote cells 

 might never be changed. Probably about one-eighth of the whole 



